Our aim is to conduct a series of theoretical and computational studies to discover what inferences can, in principle, be made concerning the forces acting on natural populations by using data on genotypic frequencies. A considerable volume of data has been accumulated during the last ten years on genotypic frequencies on natural populations - from spatially separated localities, at different time points in the same locality, or sometimes from both. Many geneticists claim that their data indicates that pressures of a particular type are operating: in our view, such claims are often premature since no overall study has yet been attempted to account for genotypic frequency configurations of various kinds. We hope to explore in particular the following problems: (1) what inferences can be made about the nature of selective forces from such data, (2) how important are historical factors in determining genotypic frequencies and whether we can distinguish between models for populations which have reached stochastic equilibrium, (3) deriving optimal statistical methods for distinguishing various forms of selection, and optimal statistical estimators of selective parameters, (4) assessing the relative importance of selective and stochastic phenomena in explaining observed genotypic frequencies, (5) computing significance tables for the test statistics derived under (3) above.